1. Field of the Invention
The present invention relates to balanced acoustic wave filter devices having balance-to-unbalance conversion functions, and particularly relates to an acoustic wave filter device in which arrangement of ground lines and ground terminals on a piezoelectric substrate is improved, and a composite filter including the acoustic wave filter.
2. Description of the Related Art
In recent years, general cellular phones have been manufactured to include a plurality of communication systems. Therefore, in order to reduce adverse effects among the communication systems, there is a strong demand for increasing an out-of-band attenuation amount. Furthermore, for miniaturization of cellular phones, a small-sized dual filter chip formed by two filter elements on a piezoelectric substrate has been developed.
As an example of such a dual filter chip, WO2006/003787 described below discloses a balanced acoustic wave filter device including first and second surface acoustic wave filter elements arranged on a piezoelectric substrate.
FIG. 13 is a plan view schematically illustrating a balanced acoustic wave filter device disclosed in WO2006/003787.
A balanced acoustic wave filter device 1001 has an electrode configuration on a piezoelectric substrate 1002 as shown in FIG. 13. In this configuration, first and second surface acoustic wave filters 1011 and 1011A are formed.
The first surface acoustic wave filters 1011 is a balanced acoustic wave filter which includes an unbalanced terminal 1013 and first and second balanced terminals 1014 and 1015 and which has a balanced-to-unbalanced conversion function. The surface acoustic wave filter 1011 has first and second 3-IDT longitudinally-coupled surface acoustic wave filter elements 1017 and 1018 which are connected to the unbalanced terminal 1013 through a single-port surface acoustic wave resonator 1016. The surface acoustic wave filter elements 1017 and 1018 are connected to first and second balanced terminals 1014 and 1015, respectively, through single-port surface acoustic wave resonators 1019 and 1020, respectively.
The second surface acoustic wave filter 1011A is configured similarly to the first surface acoustic wave filter 1011. Therefore, a description of the first surface acoustic wave filter 1011 is used for a description of the second surface acoustic wave filter 1011A by adding a character “A” to reference numerals used for the description of the first surface acoustic wave 1011.
In the acoustic wave filter device having the balanced-unbalanced conversion function, good balancing of signals between first and second balanced terminals 1014A and 1015A is required. Therefore, not only IDT electrodes and electrodes serving as reflectors formed on the piezoelectric substrate 1002 but also terminals to which input signals and ground potentials are supplied and connection lines for wiring are symmetrically arranged with respect to a virtual line passing through the center of the piezoelectric substrate 1002 to the greatest extent possible.
However, as a filter chip is miniaturized, the margin for wiring on the piezoelectric substrate 1002 is reduced.
When the acoustic wave filter device 1001 is manufactured, ground lines and ground terminals are connected to one another at a plurality of portions as much as possible. For example, in FIG. 13, a ground terminal is arranged in a portion indicated by an arrow mark A. This ground terminal is electrically connected to ground terminals of IDTs connected to the balanced terminals 1014 and 1015 of the first surface acoustic wave filter 1011 and terminals connected to ground potentials of IDTs connected to the balanced terminals 1014A and 1015A of the second surface acoustic wave filter 1010. That is, in the first and second surface acoustic wave filters 1011 and 1011A, the terminals to be connected to the ground potentials, the terminals being included in the IDTs connected to the balanced terminals, are connected to one another and further connected to a common ground terminal. In this case, the ground terminal indicated by the arrow mark A is arranged at the center between the first and second surface acoustic wave filters 1011 and 1011A. With this configuration, a symmetric characteristic is improved, and deterioration of balancing is prevented.
However, it becomes apparent that in the conventional acoustic wave filter device 1001 configured as described above, in an attenuation characteristic in the vicinity of pass band in an out-of-band of the first and second surface acoustic wave filters 1011 and 1011A, the balancing is deteriorated.